Motor axon loss is a cardinal symptom of amyotrophic lateral sclerosis (ALS). Axon loss can be driven by a genetically encoded program in which the axon survival factors NMNAT2 and STMN2 inhibit the activity of the axon destruction factor SARM1. Recent data suggest that this program of axon self-destruction may contribute to pathology in ALS. First, aggregation of TDP-43, a hallmark of most ALS cases, results in the selective loss of mRNA encoding functional STMN2, a key axon survival factor. Second, loss of SARM1 suppresses some neurodegenerative phenotypes in a mouse ALS model that expresses pathogenic human TDP-43. Here we investigate the contribution of this axon degeneration pathway to ALS. We have defined the mechanism of action of SARM1, demonstrating that it is the founding member of a new class of NAD-cleaving enzymes. SARM1 enzyme activity is normally held in check via an autoinhibitory domain. Injury- or disease- induced loss of NMNAT2 and STMN2 disinhibits SARM1, leading to rapid NAD+ depletion, metabolic catastrophe, and axon fragmentation. Our structure-function studies of the SARM1 protein have identified mutations with a range of consequences, from constitutively active variants that promote cell death and axon loss, to dominant negative variants that are neuroprotective. These findings imply that human variants may exist that either promote or protect against neurodegeneration, and that understanding the phenotypic consequences of genetic variation requires functional studies of enzyme activity. In support of this hypothesis, we have identified several rare SARM1 variants in ALS patients, but not in controls, that have constitutive NADase activity and promote neuron death and axon loss. These variants also cause motor dysfunction and paralysis when expressed in the mouse CNS, suggesting that activating SARM1 mutations may contribute to ALS pathogenesis. Here we propose to define the function of SARM1 variants from ALS patients, controls, and the general population. These studies will allow us to categorize SARM1 variants as putatively pro-degenerative, neuroprotective, or neutral. In parallel, we will dissect the contribution of variation in components of the programmed axon destruction pathway to ALS phenotypes, alone and in combination with known ALS genetic risk-factors, in motor neurons differentiated from human induced pluripotent stem cells (iPSCs). Finally, we will investigate neurodegeneration in a mouse knock-in model carrying a Sarm1 allele equivalent to a pro- degenerative allele found in ALS patients, alone and in combination with a SOD1 model, based on a specific patient genotype that we identified. We will attempt to suppress ALS phenotypes with SARM1 inhibition via a proven gene therapy approach and with experimental small molecule inhibitors. Results of these studies will establish the relationship between the SARM1-mediated axon destruction program and ALS, and build the foundation to develop axoprotective therapeutics to treat this devastating disease.
Amyotrophic lateral sclerosis (ALS) involves the loss of axons, including the essential connections between spinal cord neurons and muscles. The SARM1 enzyme is the central regulator of axon maintenance that triggers neurons to actively dismantle their axons. We have shown that mutated versions of the SARM1 gene found in some ALS patients can kill neurons and paralyze mice. Here we investigate the role of the SARM1 axon degeneration pathway in ALS in search of a therapy.
